Inverse feedback circuit



Nov. 4, 1941. D. E. FOSTER INVERSE FEEDBACK CIRCUIT Filed March 30, 1940 FREQUENCY mm mE wm N Mi L Patented Nov. 4, 1941 2,261,619 INVERSE FEEDBACK CIRCUIT Dudley E. Foster, South Orange, N. J., assignor-to' Radio Corporation of America, a corporation of Delaware Application March 30, 194.0, Serial" No. 326,913

3 Claims.

The present invention relates to vacuum tube amplifiers of the type in which inverse or negative feedback is employed, and more particularly to an improved amplifier circuit of this type wherein the range of frequency over which there is obtained a constant degree of feedback is extended.

It'is known in the prior art to employ a cathode impedance, included in both the input-and output circuits of an amplifier, for producing degeneration or negative feedback. The efiect of degenerative feedback is to extend somewhat the frequency range of an amplifier, decrease non-linear distortion and to increase the input impedance of the amplifier. In the prior art, particularly-that directed towards obtaining an increased input impedance, or avalue of input impedance which is substantially constant despite variations in grid bias of the amplifiertube, an unbypassed resistor has been used as the degenerative impedance connected to the cathode. A resistor is satisfactory when the frequencies it is desired to degenerate are low or when the frequency range to be degenerated is small. However, when constant degeneration is desired over a wide range of frequencies the effect of the inherent cathode to ground capacitance of the tube cannot'be disregarded despite the fact that such capacitances are usually only a few micromicrofarads. They may be, for example, of the order of 5 to 20 ILlLf.

I have found that with degenerative amplifiers of the type above mentioned the degree of feedback falls off to an appreciable extent with increasing frequencies. This is brought about by the shunting effect of the inherent cathode to ground capacity.

It is therefore one of the main objects of my invention to extend the frequency range over which a good degree of feedback is obtained, and I accomplish this by the use of an inductance coil in series with the cathode resistor which tends to eliminate, minimize or compensate for the effect of the inherent capacity which is effective between cathode and ground.

Another object of my invention is to vary the input impedance as well as the gain of the amplifier in a desired manner by suitable choice of values for the resistive and inductive components constituting the degenerative impedance.

The novel features characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and mode of operation together with additional objects and advantages thereof will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 discloses an inverse or negative feedback circuit embodying my invention, and

Fig. 2 are plots of several curves which-show the variation with frequency of the input impedance of an amplifier. circuit embodying my invention as compared to the conventional amplifier circuit.

Referring now to Fig. 1, the amplifier comprises an electron discharge tube T shown as of the pentode type although a triode or a screen grid tube may equally as well be employed. A tuned input circut LV1C1 is connected between the signal control grid Gs andbathode K and a similarly tuned output circuit Ls"C is connected between the plate or anode P and cathode. Connected to the cathode are the serially arranged resistor R2 and inductance coil L2 which are included in the common portion of'theinput and output circuits. The capacity inherent between the cathode K and ground is represented by the dotted line condenser Cs.

The degenerative effect on the be expressed as ajchange ineffective mutual conductance of the tube,- as will now be demonstrated. Employing the following symbols:

Sim mutual conductance of tube without degeneration,

Sm, effective mutual conductance of [tube with degeneration, I j Y.

Z1, impedance of output load, L3-'Cs,

, input voltage across L1C1,

e input voltage between signal grid Gs and cathode K,

as, output voltage across La-Cs,

Zk, impedance connected to cathode K,

Kim, input impedance of tube without degeneration (i. e. Z1 =0), and

Z'ln, input impedance of tube with degeneration,

then, if the plate impedance of the tube is high Similarly, if the input current be i1,

tube gain may The factor 1+SmZk or its reciprocal is thus seen to be a measure of the degeneration in so far as it affects gain or input impedance. Since Sm is independent of frequency if Zk be likewise independent of frequency over the desired range the degeneration will be independent of frequency over that range.

When a cathode resistor only is used for degeneration Zk will vary inappreciably over the range where the inherent cathode to ground capacity reactance of the tube is very large compared to R2. However, when the capacity reactance of C2, here designated X0 becomes appreciable compared with R2, Zk decreases as shown by curve a of Fig. 2.

Now, if inductance L2 be added in series with R2 the effect of C2 on Z1; may be nullified up to a desired limiting frequency. Specifically if R: be made equal to X0 at the highest frequency for which uniform or constant degeneration is desired and the reactance of L2, 1. e. XL be made equal to at that same frequency, Zk will be invariable with frequency up to that limiting frequency value. The value of Zr will under these conditions be as shown by curve h of Fig. 2. If L2 be made larger Zk will then vary with frequency as shown by curve 0 of Fig. 2.

Therefore, by discrete choice of L2 in comparison with R2 and C2, the effect of C2 on Zk may be nullified, or as is shown in curve 0 even reversed in direction of variation with frequency.

W le I have shown and described a pr ferred embodiment of my invention, it, will be understood that modifications and changes may be made without departing from the spirit and scope of the invention, as will be understood by those skilled in the art.

What I claim is:

1. An amplifier circuit comprising an electron discharge tube having cathode, control grid and plate electrodes, a resistor connected to the oathode and included in both the grid to cathode and plate to cathode circuits whereby degenerative feedback occurs, and an inductance coil included in the cathode circuit resonating with the capacity inherent between cathode and ground at the high end of the frequency range of the amplifier, whereby a substantially constant degree of degeneration up to substantially the limit of the frequency range is obtained.

2. A high frequency amplifier circuit operating over a desired frequency range, comprising an electron discharge tube having at least cathode, control grid and plate electrodes, a resistor and an inductance connected in series in the cathode lead of said tube and included in both the control grid to cathode and plate to cathode circuits whereby degenerative feedback occurs,

- said resistor having a value equal to the reactance of the inherent capacitance existing between cathode and ground at the highest frequency of the desired operating range, and said inductance having a reactance of half that of the inherent capacitance at the said highest frequency.

3. A degenerative amplifier stage comprising an impedance in the cathode circuit thereof, the impedance consisting of a capacitance between cathode and ground and in parallel therewith a series arrangement of a resistor and an inductance, the magnitudes of said resistor, inductance and capacitance being so chosen that the resultant cathode circuit impedance is substantially constant over a desired frequency range, said magnitudes having the relation 211 and XC =R,

where R2 is the value of the resistor and X1. and Xe are respectively the reactance of the inductance and the reactance of the capacity inherent between cathode and ground at substantially the highest frequency in the desired frequency range.

DUDLEY E. FOSTER. 

